1. Field of the Invention
The present invention relates to an integrated services digital network (ISDN) system according to recommendations of the CCITT (International Telegraph and Telephone Consultative Communication) and, in particular, to subscriber line multiplexing for multiplexing signals, particularly, control signals on a plurality of subscriber lines on a high rate digital transmission line extending to an ISDN digital switch.
2. Description of the Prior Art
In the ISDN system, a plurality of subscriber lines extend from an exchanging office to user areas. Each subscriber line terminates to a network termination (NT) unit. A user bus is connected to the NT unit and extended in the user area. One or more user's equipments or terminal equipments (TEs) such as a digital telephone set or sets (which will only be called telephone sets hereinafter), a facsimile set or sets, a personal computer set or sets, and the like.
The CCITT recommendations I series prescribes protocol of user-network connection or interface in connection with seven classified layers which are numbered by 1 through 7. For example, layer 1 is directed to physical connecting conditions such as transmission rate, pin number of connectors and others, layer 2 being for data link, layer 3 for call control from a call setup to call release completion.
According to the I series, communication information and control signal are transmitted through different channels which are multiplexed in a time division fashion. The communication information channel is called a B channel and the control information channel is referred to as a D channel. It is possible to transmit communication information through the D channel.
As a basic one of user-network interface structures, a basic interface is determined which comprises two B channels of a transmission rate of 64 kbps which are separately called B1 and B2 channels and a single D channel of a transmission rate of 16 kbps.
Another user-network interface structure is called a primary rate interface which comprises 23 B channels of 64 kbps and a single D channel of 64 kbps.
As will be noted from the above description, the single D channel is commonly used by a plurality of TEs connected in the user bus.
In connection with the layer 2 for data link, a plurality of logical connections or links are made on the D channel by use of LAPD (Link Access Procedure on D channel). Various control information in layer 2 and layer 3 can be transmitted through the links. A frame format of the LAPD is in conformity with the HDLC (High-level Data Link Control Procedure) and comprises a start flag, an address field, a control field, an information field, a frame check sequence and a stop flag. The frame format will later be described in detail with reference to the drawings.
The address field is rewritable to identify each one logical link. The control field is used for type identification of the LAPD frame itself. There are defined a number of types for the LAPD frame. For example, I (Information) frame is used for information transfer in a multiframe mode and UI (Unnumbered Information) frame is used for information transfer in an unacknowledged information transfer mode.
Transfer of call control information in the layer 3 is performed by use of the I frame or the UI frame. The call control information is inserted in the information field in each of the I and UI frames as a layer 3 message. A format of the layer 3 message comprises a common information portion and an individual information portion. The common information portion comprises a protocol discriminator for identifying the layer 3 message, a call reference for referring to a call, and a message type. There are a number of types for the layer 3 message, for example, SETUP for identifying a call request, REL COMP for identifying notice of completion of channel disconnection and release completion of a call reference, and others.
A large number of subscriber lines are extended from an exchange office having an ISDN digital switch to various users. When a user such as a company has a plurality of subscriber lines, it is advantageous to multiplex those subscriber lines into a single transmission line through which signals on the plurality of subscriber lines are transmitted as a TDM (time division multiplexed) signal. That is, B channel signals and D channel signals on a plurality of subscriber lines are time-division multiplexed and are transmitted through the signal transmission line.
Providing that each subscriber line is dependent on the basic interface having two B channels of 64 kbps and a single D channel of 16 kbps and that a primary rate digital transmission line (1544 kbps, 23B+D channels) is used for the single transmission line, only four D channels (64/16=4) can be multiplexed.
In order to resolve the problem, the Bell Communications Research, Inc. proposed a "Statistical Multiplexing" in Bell Communications Research Technical Advisory, TA-TSY-000397, May 2, 1987 (Reference 1). According to the Statistical Multiplexing, 16 D channel signals on subscriber lines of the basic interface are statistically multiplexed into a D channel on the primary rate transmission line. In order to identify each subscriber line and to identify multiplex, the identification information is tagged to the D channel information on each subscriber line to form a new D channel signal which is transmitted to the transmission line in the statistical multiplexing. The new D channel signal will later be described in detail in connection with the drawings.
According to the statistical multiplexing, an increased number of D channels can be multiplexed in comparison with the conventional time division multiplexing. However, a data link is still set up between a TE on one subscriber line and an ISDN switch or exchanger so that it is impossible to reduce load of the ISDN switch.
In multiplexing of a plurality of subscriber lines, it is impossible to handle those lines which belong to different exchange offices.
In another ISDN system, a private line or a privately used line is used. It is also impossible in the multiplexing to handle the private line and subscriber lines which belong to an exchange office together.